The field of the invention relates generally to control systems for pressure vessels, and more particularly, to a method and system for calibrating a transient model for use in controlling a pressure vessel.
Pressure vessels are widely used in various power plants and related industries. The design principle uses the difference in density between cooler water in the downcomer and the steam/water mixture in the riser to drive the steam/water mixture through the tubes. The pressure vessel drum separates steam from water. Water enters the riser tube, is heated, and undergoes a transition from a single-phase liquid to a mixture of saturated liquid and steam. As heat input increases, the proportion of steam vapor in the riser tube increases.
A high-priority challenge to the control engineer is the ability to control the water level in the drum very precisely. When the water level gets too high, it can result in water carryover into the superheater or turbine, potentially causing damage or outages in the turbine or pressure vessel. A level that is too low can expose the water tubes to high heat input, causing them to crack or break. A pressure vessel trip interlock is supposed to prevent these types of damage, but pressure vessel trips can take considerable time to clear, during which the expensive production equipment is often forced to sit idle.
As a result, several methods of control systems have been designed to model the physics of the pressure vessel and therefore to give the control engineer the key determinants to understand the dynamics of the vessel. Based on such control systems, the control engineer can determine the water level and pressure of the system. A prevailing model is the Astrom-Bell 5-states drum model.
This model captures the key properties of pressure vessels over a wide operating range. The model also pays particular attention to modeling the pressure vessel drum water level dynamics. As stated above, this is a crucial consideration. The model consists of five state variables—the total water volume, the drum pressure, the steam quality at the riser exit, the steam volume below water level, and the downcomer exit flow rate.
Most known control systems adapting the Astrom-Bell 5-states model for pressure vessels use an initial pressure and level reading for the pressure vessel and then generate a calibrated model based upon the state variables above.
These systems are prone to inaccuracy because the parameters affecting level and pressure used are held as constants. In practice, if the physics captured by the model is inadequate or the physical system behavior changes from degradation because of tube fouling or scaling, these values may change and require a re-evaluation of the model to accurately predict level and pressure dynamics. In the absence of such re-evaluation, the system may become inadequately controlled.
Inadequate pressure vessel control can have significant practical consequences. It can result in increased maintenance costs, downtime, and potential damage to the pressure vessel.